980 MPA-GRADE FULL-BAINITE ULTRA-HIGH HOLE EXPANSION STEEL AND MANUFACTURING METHOD THEREFOR

Abstract

A 980 MPa-grade full-bainite ultra-high hole expansion steel and a manufacturing method therefor. The hole expansion steel has the following chemical compositions in percentage by weight: 0.05-0.10% of C, Si≤2.0%, 1.0-2.0% of Mn, P≤0.02%, S≤0.003%, 0.02-0.08% of Al, N≤0.004%, 0.1-0.5% of Mo, 0.01-0.05% of Ti, O≤0.0030%, the remainder being Fe, and other inevitable impurities. The ultra-high hole expansion steel in the present invention has yield strength ≥800 MPa, tensile strength ≥980 MPa, and a hole expansion rate up to 60% or more, and can be applied in the parts of chassis components such as a control arm and an auxiliary frame, which require high strength thinning and complex forming, of passenger vehicles.

Claims

1. A 980 MPa-grade fully bainitic ultra-high-hole-expandability steel, comprising the following chemical components in weight percentages: C 0.05-0.10%, Si≤2.0%, Mn 1.0-2.0%, P≤0.02%, S≤0.003%, Al 0.02-0.08%, N≤0.004%, Mo 0.1-0.5%, Ti 0.01-0.05%, O≤0.0030%, and a balance of Fe and other unavoidable impurities.

2. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 1, further comprising one or more elements selected from the group consisting of Cr≤0.05%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V≤0.05%, Cu≤0.05%, and Ni≤0.5%.

3. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 2, further comprising Cr≤0.5% and/or B≤0.002%.

4. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 1, wherein C: 0.06-0.09%.

5. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 1, wherein Mn: 1.4-1.8%.

6. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 1, wherein S is controlled to have a content of 0.0015% or lower, and/or N is controlled to have a content of 0.003% or lower.

7. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 1, wherein Al: 0.02-0.05%.

8. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 1, wherein Ti: 0.01-0.03%, and/or Mo: 0.15-0.35%.

9. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 1, wherein the ultra-high-hole-expandability steel has a microstructure of full bainite.

10. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 1, wherein the ultra-high-hole-expandability steel has a yield strength of ≥800 MPa, a tensile strength of ≥980 MPa, a transverse elongation A.sub.50 of ≥10%, a hole expansion ratio of ≥60%, and has passed cold bending test (d≤4a, 180°).

11. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 10, wherein the ultra-high-hole-expandability steel has a yield strength of ≥850 MPa, a tensile strength of ≥1020 MPa, a transverse elongation A.sub.50 of ≥10%, a hole expansion ratio of ≥70%, and has passed cold bending test (d≥4a, 180°); wherein the ultra-high-hole-expandability steel has an impact toughness at −40° C. of ≥50 J.

12. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 10, wherein the ultra-high-hole-expandability steel has a yield strength of ≥830 MPa, a tensile strength of ≥1000 MPa, a transverse elongation A.sub.50 of ≥10%, a hole expansion ratio of ≥70%, and has passed cold bending test (d≤4a, 180°); wherein the ultra-high-hole-expandability steel has an impact toughness at −40° C. of ≥60 J.

13. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 10, wherein the ultra-high-hole-expandability steel has a yield strength of ≥900 MPa, a tensile strength of ≥1040 MPa, a transverse elongation A.sub.50 of ≥10%, a hole expansion ratio of ≥65%, and has passed cold bending test (d≤4a, 180°); wherein the ultra-high-hole-expandability steel has an impact toughness at −40° C. of ≥40 J.

14. A method for manufacturing the 980 MPa-grade fully bainitic ultra-high hole-expandability steel according to claim 1, comprising the following steps: 1) Smelting, casting wherein the components according to claim 1 are subjected to smelting in a converter or electrical furnace, secondary refining in a vacuum furnace, and casting to form a cast blank or ingot; 2) Reheating of the cast blank or ingot, wherein a heating temperature is 1100-1200 ° C.; and a holding time is 1-2 hours; 3) Hot rolling wherein an initial rolling temperature is 950-1100° C.; wherein 3-5 passes of heavy reduction rolling is performed at a temperature of 950° C. or higher with an accumulated deformation rate of ≥50%, to obtain an intermediate blank; wherein the intermediate blank is held till 930-950° C., and then subjected to 5-7 passes of finishing rolling with an accumulated deformation rate of ≥70%, wherein a final rolling temperature is 800-930° C.; (4) Cooling wherein air cooling is performed for 0-10 seconds to allow for dynamic restoration and dynamic recrystallization, and then water cooling is performed, wherein the strip steel is water cooled to a temperature range of bainite transformation, i.e. in the range of B.sub.s to B.sub.f, at a cooling rate of ≥10° C./s, wherein after coiling to obtain a steel coil, air cooling is utilized to cool the steel coil to room temperature; (5) Pickling wherein a moving speed of the strip steel is adjusted within a range of 30-100 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤2%; wherein the strip steel is then subjected to rinsing, surface drying, and oiling.

15. The method for manufacturing the 980 MPa-grade fully bainitic ultra-high hole-expandability steel according to claim 14, wherein in step 5), after the pickling, the rinsing is carried out at a temperature in a range of 35-50° C., and the surface of the strip steel is dried at 120-140° C., followed by oiling.

16. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 2, wherein Nb and V each have a content of ≤0.03%; Cu and Ni each have a content of ≤0.3%; Cr has a content of 0.2-0.4%; B has a content of 0.0005-0.0015%; and Ca has a content of ≤0.002%.

17. The 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to claim 10, wherein the ultra-high-hole-expandability steel has an impact toughness at −40° C. of ≥40 J.

18. The method for manufacturing the 980 MPa-grade fully bainitic ultra-high hole-expandability according to claim 14, wherein: in step (3), 3-5 passes of heavy reduction rolling is performed at a temperature of 950° C. or higher with an accumulated deformation rate of ≥70% to obtain an intermediate blank; the intermediate blank is subjected to 5-7 passes of finishing rolling with an accumulated deformation rate of ≥80%; in step (4), the cooling rate is 10-60° C./s; the coiling temperature is 410-550° C.

19. The method for manufacturing the 980 MPa-grade fully bainitic ultra-high hole-expandability steel according to claim 14, wherein the 980 MPa-grade fully bainitic ultra-high-hole-expandability steel: (1) further comprises one or more elements selected from Cr≤0.5%, B≤0.002%, Ca≤0.005%, Nb≤0.06%, V≤0.05%, Cu≤0.5%, and Ni≤0.5%; or (2) further comprises Cr≤0.5% and/or B≤0.002%; or (3) comprises 0.06-0.09% of C; or (4) comprises 1.4-1.8% or Mn; or (5) comprises S with a content of 0.0015% or lower, and/or N with a content of 0.003% or lower; or (6) comprises 0.02-0.05% of Al; or (7) comprises Ti: 0.01-0.03%, and/or Mo: 0.15-0.35%; or (8) has a microstructure of full bainite.

20. The method for manufacturing the 980 MPa-grade fully bainitic ultra-high hole-expandability steel according to claim 14, the 980 MPa-grade fully bainitic ultra-high-hole-expandability steel has a yield strength of ≥800 MPa, a tensile strength of ≥980 MPa, a transverse elongation A.sub.50 of ≥10%, a hole expansion ratio of ≥60%, and has passed cold bending test (d≤4a, 180°).

Description

DESCRIPTION OF THE DRAWINGS

[0058] FIG. 1 is a process flow chart of the method for manufacturing a 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to the present disclosure.

[0059] FIG. 2 is a schematic view showing the rolling process in the method for manufacturing a 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to the present disclosure.

[0060] FIG. 3 is a schematic view showing the cooling process in the method for manufacturing a 980 MPa-grade fully bainitic ultra-high-hole-expandability steel according to the present disclosure.

[0061] FIG. 4 is a typical metallographical photo of the ultra-high-hole-expandability steel of Example 3 according to the disclosure.

[0062] FIG. 5 is a typical metallographical photo of the ultra-high-hole-expandability steel of Example 5 according to the disclosure.

[0063] FIG. 6 is a typical metallographical photo of the ultra-high-hole-expandability steel of Example 7 according to the disclosure.

DETAILED DESCRIPTION

[0064] Referring to FIG. 1-FIG. 3, the method for manufacturing the 980 MPa-grade fully bainitic ultra-high hole-expandability steel according to the present disclosure comprises the following steps: [0065] 1) Smelting, casting [0066] wherein the above components are subjected to smelting in a converter or electrical furnace, secondary refining in a vacuum furnace, and casting to form a cast blank or ingot; [0067] 2) Reheating of the cast blank or ingot, wherein a heating temperature is 1100-1200° C.; and a holding time is 1-2 hours; [0068] 3) Hot rolling [0069] wherein an initial rolling temperature is 950-1100° C.; wherein 3-5 passes of heavy reduction rolling is performed at a temperature of 950° C. or higher with an accumulated deformation rate of ≥50% to obtain an intermediate blank; wherein the intermediate blank is held till 930-950° C., and then subjected to 5-7 passes of finishing rolling with an accumulated deformation rate of ≥70%, wherein a final rolling temperature is 800-930° C.; [0070] 4) Cooling [0071] wherein air cooling is performed for 0-10 seconds to allow for dynamic restoration and dynamic recrystallization, and then water cooling is performed, wherein the strip steel is water cooled to a temperature range of bainite transformation, i.e. B.sub.s, to B.sub.f, at a cooling rate of ≥10° C./s; wherein after coiling to obtain a steel coil, air cooling (cooling rate >20° C./h) is utilized to speed up cooling of the steel coil until room temperature; [0072] 5) Pickling [0073] wherein a moving speed of the strip steel is adjusted within a range of 30-100 m/min during pickling; a pickling temperature is controlled at 75-85° C., and a tension leveling rate is controlled at ≤2%; wherein the strip steel is then subjected to rinsing at a temperature in a range of 35-50° C., surface drying at a temperature in a range of 120-140° C., and oiling.

[0074] The compositions of the Examples of the ultra-high-hole-expandability steel according to the present disclosure are shown in Table 1. The production process parameters for the Examples of the steel according to the present disclosure are listed in Table 2 and Table 3, wherein the thickness of the steel blank in the rolling process is 230 mm The mechanical performances of the Examples of the steel plates according to the present disclosure are listed in Table 4. The tensile performances (yield strength, tensile strength, elongation) were tested in accordance with International Standard ISO6892-2-2018; the hole expansion ratio was tested in accordance with International Standard ISO16630-2017; and the impact energy was tested in accordance with International Standard ISO14556-2015.

[0075] As it can be seen from Table 4, the yield strength of the steel coils is ≥800 MPa, while the tensile strength is ≥980 MPa, and the elongation is usually ≥10%. The impact energy is relatively stable. The impact energy at −40° C. is ≥40J, and the hole expansion ratio is ≥60%. As it can be seen from the above Examples, the 980 MPa high-strength steel according to the present disclosure exhibits excellent matching of strength, plasticity, toughness and hole expandability. It is especially suitable for parts that require high strength, reduced thickness, hole expansion and flanging forming, such as a control arm in an automobile chassis structure. It can also be used for complexly-shaped parts such as wheels that need hole flanging. Therefore, it has broad application prospects.

TABLE-US-00001 TABLE 1 (unit: weight %) Ex. C Si Mn P S Al N Mo Ti Cr B Ca Nb V Cu Ni O 1 0.055 0.08 1.48 0.013 0.0028 0.055 0.0029 0.35 0.030 0.28 0.0010 0.002 / / 0.20 0.10 0.0028 2 0.084 1.55 1.77 0.009 0.0010 0.040 0.0038 0.18 0.049 0.36 0.0012 / 0.060 / / / 0.0025 3 0.073 0.66 1.02 0.014 0.0020 0.079 0.0033 0.26 0.020 / 0.0020 / / 0.04 0.30 0.20 0.0022 4 0.090 1.08 1.67 0.010 0.0027 0.025 0.0025 0.44 0.046 0.31 0.0015 0.001 / 0.03 / 0.50 0.0027 5 0.060 0.38 1.26 0.008 0.0012 0.052 0.0036 0.12 0.018 0.50 0.0018 0.003 / / / 0.30 0.0026 6 0.098 0.22 1.83 0.011 0.0024 0.033 0.0030 0.49 0.028 0.30 0.0005 / 0.030 / 0.50 / 0.0029 7 0.051 1.88 1.39 0.015 0.0009 0.022 0.0034 0.25 0.035 0.32 0.0018 0.005 0.015 0.02 / / 0.0021 8 0.077 1.24 1.99 0.009 0.0025 0.057 0.0022 0.31 0.010 0.20 / / / 0.05 / / 0.0024

TABLE-US-00002 TABLE 2 Initial Accumulated Intermediate Accumulated Final Air Cooling Heating rolling deformation blank deformation rolling cooling Water Steel Coiling rate temper- Holding temper- rate during temper- rate during temper- time cooling plate temper- after ature time ature rough ature finishing ature (s) rate thickness ature coiling ° C. h ° C. rolling % ° C. rolling % ° C. s ° C./s mm ° C. ° C./h Ex. 1 1160 1.3 1000 90 930 84 860 5 30 3.6 450 30 Ex. 2 1100 2.0 960 70 940 97 800 2 60 2.2 500 52 Ex. 3 1140 1.6 1080 85 950 87 870 9 10 4.5 410 28 Ex. 4 1200 1.0 1020 80 935 94 830 0 40 2.6 480 43 Ex. 5 1120 1.9 950 75 940 93 930 3 15 3.8 520 58 Ex. 6 1180 1.1 1100 83 950 92 850 0 50 3.0 430 25 Ex. 7 1130 1.8 1050 80 935 88 900 7 25 5.5 550 63 Ex. 8 1150 1.5 980 86 945 85 920 4 55 4.8 460 40

TABLE-US-00003 TABLE 3 Moving speed of strip steel Pickling Tension Rinsing Drying during temper- leveling temper- temper- pickling ature rate ature ature m/min ° C. % ° C. ° C. Ex. 1 50 82 1.2 40 135 Ex. 2 30 76 1.8 35 120 Ex. 3 55 75 0.5 47 128 Ex. 4 45 80 1.6 42 140 Ex. 5 100 77 0.8 50 133 Ex. 6 85 79 2.0 37 125 Ex. 7 60 81 1.4 41 134 Ex. 8 70 83 1.0 38 130

TABLE-US-00004 TABLE 4 Mechanical performances of steel plates Yield Tensile Elon- Hole −40° C. strength strength gation expansion impact MPa MPa % ratio % energy J Ex. 1 913 1048 11.0 65 53 Ex. 2 960 1070 11.5 83 45 Ex. 3 967 1070 10.5 90 70 Ex. 4 902 1044 11.0 70 67 Ex. 5 885 1047 11.5 77 73 Ex. 6 838 1043 10.0 68 66 Ex. 7 832 1037 10.5 75 79 Ex. 8 858 1018 11.5 80 88

[0076] Note: The impact energy is obtained by converting the measured impact energy of a sample having an actual thickness into the impact energy of a standard sample of 10*10*55 mm in proportion based on equivalent effect.